The present invention relates to a pneumograph, which is an instrument for sensing and measuring respiratory movements. More particularly, the present invention relates to the design and operation of a capsule pneumograph utilized for the detection of respiratory movements in subjects during polysomnographic sleep monitoring.
The pneumograph has been in use for years, with its usual construction being in the form of a rubber bellows connected to a chain or strap that encircles the chest so that respiratory movements expand and contract the bellows. The expansion and contraction of the bellows causes the pressure within the bellows to fall and rise in proportion to the chest excursion. The variations in pressure are sensed by a pressure transducer connected to the bellows by a tube.
The bellows pneumograph produces adequate results where an application requires knowledge of the phase and frequency, but not the precise depth, of respiration. Specifically, since this pneumograph encircles only the chest or abdomen, it can respond to movements in only one direction of a system that has two degrees of freedom. Also, the bellows pneumograph must be used with subjects in relatively fixed positions because uncontrolled body movements may increase or decrease the resting tension in the bellows, thus, changing the base line of the recording or even driving the transducer out of its useful range. If the strap slips, the respiratory movements may cease to be recorded at all. Another disadvantage of a bellows pneumograph is that a subject in a recumbent position may experience discomfort from lying on the chain, the strap or the bellows and, thus, have his freedom of movement limited. For these reasons, the bellows pneumograph has found little acceptance in sleep laboratories.
The prior art also includes the recent development of an apnea monitor that uses a pressure capsule as a motion sensor for use in the monitoring of infants. This development is described in "Long Term Respiration Monitoring in Infants--A Comparison of Impedance and Pressure Capsule Monitors" by R. Railton et al, Clin. Phys. Physiol. Meas., 1983; 4(1); 91-94.